As is known, essentially unsaturated diene rubber vulcanisates, both natural and synthetic, due to the presence of double bonds on their molecular chains, are liable, if they are not protected, to deteriorate more or less rapidly after prolonged exposure to the atmosphere, due to known oxidation mechanisms. These complex mechanisms have been restated, for example, in the patent documents WO 99/02590 and WO 99/06480. They result, subsequent to cleavages of these double bonds and to the oxidation of the sulphur bridges, in a stiffening and a weakening of the vulcanisates, which damage is additionally accelerated under the joint action of heat, by “thermal-oxidation”, or also of that of light by “photo-oxidation”.
It has been possible to gradually inhibit these oxidation phenomena by virtue of the development and marketing of various antioxidants, including in particular p-phenylenediamine (“PPD” or “PPDA”) derivatives, such as, for example, N-isopropyl-N′-phenyl-p-phenylenediamine (“I-PPD”) or N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (“6-PPD”), or quinoline derivatives (“TMQ”), simultaneously excellent antioxidants and antiozonants (see, for example, Patent Applications WO 2004/033548, WO 2005/063510 and WO 2005/133666). These antioxidants are today used systematically in diene rubber compositions, in particular in compositions for tires, in order to combat ageing and premature wear of the latter.
The well-known disadvantage of these antioxidants is that their concentration in the rubber compositions naturally decreases over time due to their very chemical function and that they have, in addition, a strong natural propensity to migrate from the regions more concentrated in antioxidant towards the regions less concentrated in antioxidant, so much so that a person skilled in the art is led to use relatively large amounts of product, which is relatively expensive and furthermore harmful to the appearance of the finished products, due to a high staining power of a large number of antioxidants, in particular p-phenylenediamine derivatives.
In order to overcome the above disadvantages and thus to further improve the protection and the resistance to ageing of tires, the proposal has in particular been made to incorporate, in these tires, additional rubber layers having a high concentration of antioxidant, operating in a way as antioxidant reservoirs capable of delivering the antioxidant with the passage of time, by migration, as a function of the degree of depletion of the adjacent regions (see, for example, the patent documents WO 2009/029114, EP 1 319 527 or U.S. Pat. No. 7,082,976).
The use of such antioxidant reservoirs, although effective, however exhibits the disadvantage of having to modify the internal structure of the tires and consequently in particular of making it more complex and more expensive to manufacture them.
Consequently, the designers of diene rubber articles, in particular tire manufacturers, are today looking for simple novel solutions which make it possible to overcome, at least in part, the abovementioned disadvantages.